Communication systems

ABSTRACT

A path selection method for use in a communication system, the system comprising at least three communication apparatuses, a particular one of said apparatuses being operable to transmit and/or receive a communication signal along at least two different communication paths, each said path being either a single-link path extending from the particular apparatus to another said apparatus directly over a single communication link therebetween, or being a multi-link path extending from the particular apparatus to said other or another said apparatus indirectly via one or more intermediate said apparatuses over a plurality of consecutive such links link-by-link along the path, and at least one said path being such a multi-link path, the method comprising: for each link along at least the or one of the multi-link paths, obtaining link-suitability information indicative of the suitability of the link concerned for transmission and/or reception; for at least the or said one of the multi-link paths, combining the link-suitability information for each link of the path concerned so as to generate path-suitability information indicative of the suitability of the path concerned for transmission and/or reception; and selecting one of said paths for transmission and/or reception in dependence upon said path-suitability information.

Currently there exists significant interest in the use of multihoptechniques in packet based radio and other communication systems, whereit is purported that such techniques will enable both extension incoverage range and increase in system capacity (throughput).

In a multi-hop communication system, communication signals are sent in acommunication direction along a communication path (C) from a sourceapparatus to a destination apparatus via one or more intermediateapparatuses. FIG. 5 illustrates a single-cell two-hop wirelesscommunication system comprising a base station BS (known in the contextof 3 G communication systems as “node-B” NB) a relay node RN (also knownas a relay station RS) and a user equipment UE (also known as mobilestation MS). In the case where signals are being transmitted on thedownlink (DL) from a base station to a destination user equipment (UE)via the relay node (RN), the base station comprises the source station(S) and the user equipment comprises the destination station (D). In thecase where communication signals are being transmitted on the uplink(UL) from a user equipment (UE), via the relay node, to the basestation, the user equipment comprises the source station and the basestation comprises the destination station. The relay node is an exampleof an intermediate apparatus (I) and comprises: a receiver, operable toreceive data from the source apparatus; and a transmitter, operable totransmit this data, or a derivative thereof, to the destinationapparatus.

Simple analogue repeaters or digital repeaters have been used as relaysto improve or provide coverage in dead spots. They can either operate ina different transmission frequency band from the source station toprevent interference between the source transmission and the repeatertransmission, or they can operate at a time when there is notransmission from the source station.

FIG. 6 illustrates a number of applications for relay stations. Forfixed infrastructure, the coverage provided by a relay station may be“in-fill” to allow access to the communication network for mobilestations which may otherwise be in the shadow of other objects orotherwise unable to receive a signal of sufficient strength from thebase station despite being within the normal range of the base station.“Range extension” is also shown, in which a relay station allows accesswhen a mobile station is outside the normal data transmission range of abase station. One example of in-fill shown at the top right of FIG. 6 ispositioning of a nomadic relay station to allow penetration of coveragewithin a building that could be above, at, or below ground level.

Other applications are nomadic relay stations which are brought intoeffect for temporary cover, providing access during events oremergencies/disasters. A final application shown in the bottom right ofFIG. 6 provides access to a network using a relay positioned on avehicle.

Relays may also be used in conjunction with advanced transmissiontechniques to enhance gain of the communications system as explainedbelow.

It is known that the occurrence of propagation loss, or “pathloss”, dueto the scattering or absorption of a radio communication as it travelsthrough space, causes the strength of a signal to diminish. Factorswhich influence the pathloss between a transmitter and a receiverinclude: transmitter antenna height, receiver antenna height, carrierfrequency, clutter type (urban, sub-urban, rural), details of morphologysuch as height, density, separation, terrain type (hilly, flat). Thepathloss L (dB) between a transmitter and a receiver can be modelled by:

L=b+10n log d  (A)

Where d (metres) is the transmitter-receiver separation, b(db) and n arethe pathloss parameters and the absolute pathloss is given byl=10^((L/10)).

The sum of the absolute path losses experienced over the indirect linkSI+ID may be less than the pathloss experienced over the direct link SD.In other words it is possible for:

L(SI)+L(ID)<L(SD)  (B)

Splitting a single transmission link into two shorter transmissionsegments therefore exploits the non-linear relationship between pathlossverses distance. From a simple theoretical analysis of the pathlossusing equation (A), it can be appreciated that a reduction in theoverall pathloss (and therefore an improvement, or gain, in signalstrength and thus data throughput) can be achieved if a signal is sentfrom a source apparatus to a destination apparatus via an intermediateapparatus (e.g. relay node), rather than being sent directly from thesource apparatus to the destination apparatus. If implementedappropriately, multi-hop communication systems can allow for a reductionin the transmit power of transmitters which facilitate wirelesstransmissions, leading to a reduction in interference levels as well asdecreasing exposure to electromagnetic emissions. Alternatively, thereduction in overall pathloss can be exploited to improve the receivedsignal quality at the receiver without an increase in the overallradiated transmission power required to convey the signal.

Multi-hop systems are suitable for use with multi-carrier transmission.In a multi-carrier transmission system, such as FDM (frequency divisionmultiplex), OFDM (orthogonal frequency division multiplex) or DMT(discrete multi-tone), a single data stream is modulated onto N parallelsub-carriers, each sub-carrier signal having its own frequency range.This allows the total bandwidth (i.e. the amount of data to be sent in agiven time interval) to be divided over a plurality of sub-carriersthereby increasing the duration of each data symbol. Since eachsub-carrier has a lower information rate, multi-carrier systems benefitfrom enhanced immunity to channel induced distortion compared withsingle carrier systems. This is made possible by ensuring that thetransmission rate and hence bandwidth of each subcarrier is less thanthe coherence bandwidth of the channel. As a result, the channeldistortion experienced on a signal subcarrier is frequency independentand can hence be corrected by a simple phase and amplitude correctionfactor. Thus the channel distortion correction entity within amulticarrier receiver can be of significantly lower complexity of itscounterpart within a single carrier receiver when the system bandwidthis in excess of the coherence bandwidth of the channel.

Orthogonal frequency division multiplexing (OFDM) is a modulationtechnique that is based on FDM. An OFDM system uses a plurality ofsub-carrier frequencies which are orthogonal in a mathematical sense sothat the sub-carriers' spectra may overlap without interference due tothe fact they are mutually independent. The orthogonality of OFDMsystems removes the need for guard band frequencies and therebyincreases the spectral efficiency of the system. OFDM has been proposedand adopted for many wireless systems. It is currently used inAsymmetric Digital Subscriber Line (ADSL) connections, in some wirelessLAN applications (such as WiFi devices based on the IEEE802.11a/gstandard), and in wireless MAN applications such as WiMAX (based on theIEEE 802.16 standard). OFDM is often used in conjunction with channelcoding, an error correction technique, to create coded orthogonal FDM orCOFDM. COFDM is now widely used in digital telecommunications systems toimprove the performance of an OFDM based system in a multipathenvironment where variations in the channel distortion can be seenacross both subcarriers in the frequency domain and symbols in the timedomain. The system has found use in video and audio broadcasting, suchas DVB and DAB, as well as certain types of computer networkingtechnology.

In an OFDM system, a block of N modulated parallel data source signalsis mapped to N orthogonal parallel sub-carriers by using an InverseDiscrete or Fast Fourier Transform algorithm (IDFT/IFFT) to form asignal known as an “OFDM symbol” in the time domain at the transmitter.Thus, an “OFDM symbol” is the composite signal of all N sub-carriersignals. An OFDM symbol can be represented mathematically as:

$\begin{matrix}{{{x(t)} = {\frac{1}{\sqrt{N}}{\sum\limits_{n = 0}^{N - 1}\; {c_{n} \cdot ^{{j2\pi}\; n\; \Delta \; {ft}}}}}},{0 \leq t \leq T_{s}}} & (1)\end{matrix}$

where Δf is the sub-carrier separation in Hz, Ts=1/Δf is symbol timeinterval in seconds, and c_(n) are the modulated source signals. Thesub-carrier vector in (1) onto which each of the source signals ismodulated cεC_(n), c=(C₀, C₁ . . . C_(N−1)) is a vector of Nconstellation symbols from a finite constellation. At the receiver, thereceived time-domain signal is transformed back to frequency domain byapplying Discrete Fourier Transform (DFT) or Fast Fourier Transform(FFT) algorithm.

OFDMA (Orthogonal Frequency Division Multiple Access) is a multipleaccess variant of OFDM. It works by assigning a subset of sub-carriers,to an individual user. This allows simultaneous transmission fromseveral users leading to better spectral efficiency. However, there isstill the issue of allowing bi-directional communication, that is, inthe uplink and download directions, without interference.

In order to enable bi-directional communication between two nodes, twowell known different approaches exist for duplexing the two (forward ordownload and reverse or uplink) communication links to overcome thephysical limitation that a device cannot simultaneously transmit andreceive on the same resource medium. The first, frequency divisionduplexing (FDD), involves operating the two links simultaneously but ondifferent frequency bands by subdividing the transmission medium intotwo distinct bands, one for forward link and the other for reverse linkcommunications. The second, time division duplexing (TDD), involvesoperating the two links on the same frequency band, but subdividing theaccess to the medium in time so that only the forward or the reverselink will be utilizing the medium at any one point in time. Bothapproaches (TDD & FDD) have their relative merits and are both well usedtechniques for single hop wired and wireless communication systems. Forexample the IEEE802.16 standard incorporates both an FDD and TDD mode.

As an example, FIG. 7 illustrates the single hop TDD frame structureused in the OFDMA physical layer mode of the IEEE802.16 standard(WiMAX).

Each frame is divided into DL and UL subframes, each being a discretetransmission interval. They are separated by Transmit/Receive andReceive/Transmit Transition Guard interval (TTG and RTG respectively).Each DL subframe starts with a preamble followed by the Frame ControlHeader (FCH), the DL-MAP, and the UL-MAP.

The FCH contains the DL Frame Prefix (DLFP) to specify the burst profileand the length of the DL-MAP. The DLFP is a data structure transmittedat the beginning of each frame and contains information regarding thecurrent frame; it is mapped to the FCH.

Simultaneous DL allocations can be broadcast, multicast and unicast andthey can also include an allocation for another BS rather than a servingBS. Simultaneous ULs can be data allocations and ranging or bandwidthrequests.

This patent application is one of a set of ten UK patent applicationsfiled on the same date by the same applicant with agent referencenumbers P106752 GB00, P106753 GB00, P106754 GB00, P106772 GB00, P106773GB00, P106795 GB00, P106796 GB00, P106797 GB00, P106798 GB00, andP106799 GB00, describing interrelated inventions proposed by the presentinventors relating to communication techniques. The entire contents ofeach of the other nine applications is incorporated herein by way ofreference thereto and copies of each of the other nine applications arefiled herewith.

In WiMAX and other networks, a mobile station (MS) may face manypossible paths to exchange information with base station (BS).Approaches should be designed for MS to select an optimal path.Especially, in mobile environment, the radio channel condition will bedynamically changing, thus a MS should dynamically select a proper relaystation (RS), or BS for handover.

The invention is defined in the independent claims, to which referenceshould now be made. Advantageous embodiments are set out in the subclaims.

Preferred features of the present invention will now be described,purely by way of example, with reference to the accompanying drawings,in which:—

FIG. 1 shows Scenario 1: BS can cover the MS. An MS will have manypossible paths to communication with BS in WiMAX relay systems. Thesystem has to choose an optimal path for the communication between MSand BS;

FIG. 2 shows Scenario 2: BSs cannot cover MS. The system has to chooseeither RS 1# or RS 2# to relay the information between MS and BSs;

FIG. 3 shows an example of calculating PoR values;

FIG. 4 shows an example of the signaling related to PoR table broadcast;

FIG. 5 shows a single-cell two-hop wireless communication system;

FIG. 6 shows applications of relay stations; and

FIG. 7 shows a single hop TDD frame structure used in the OFDMA physicallayer mode of the IEEE 802.16 standard.

FIGS. 1 and 2 illustrate scenarios of WiMAX relay systems, where theMobile Station (MS) could connect to BS directly, or ask RS 1# or RS 2#for relay. Obviously, in FIG. 1, following paths are possible for thecommunication between MS and BS:

-   -   a. MS->BS    -   b. MS->RS1#->BS    -   c. MS->RS1#->RS2->BS    -   d. MS->RS2#->BS    -   e. MS->RS2#->RS1->BS

The system shall decide an optimal path for the handover of MS. Thisdecision should be based on the link quality, QoS demand, and so on.Especially in high mobility case, the MS's status will be dynamicallychanging, thus the RS/BS selection approach will be very important tomaintain an acceptable QoS level in MS.

An MS will have many possible paths to communication with BS in WiMAXrelay systems. The system has to choose an optimal path for thecommunication between MS and BS.

The system has to choose either RS 1# or RS 2# to relay the informationbetween MS and BSs.

The previous method for MS to decide the handover is using link quality,such as CINR, which could be measured from preamble or otherpre-determined receiving sequences. However, in WiMAX relay systems, wemay encounter issues as listed below for MS's handover decision, whichare considered in this proposed patent.

-   1. Link quality metrics, such as CINR, and RSSI, are not enough for    BS/RS selection for relay systems. For example, if RS 2# has better    CINR than RS 1#, but RS 2# does not have enough available bandwidth    to afford a kind level of QoS level for MS, then MS may choose RS 1    # for relay;-   2. If considering multiple metrics for BS/RS selection, the overhead    of signaling may be increased;-   3. The relay path will be more than two hops, such as the listed    path c, and e. Therefore, how to measure link quality of multi-hop    path, and how to signaling each other with small increasing overhead    are the possible issues.-   4. To support high mobility users, the metrics for BS/RS selection    should be efficiently refreshed.

Details of the Proposed BS/RS Selection Scheme Definition of theSelection Metric, Potent of Relay (PoR):

Each path will have a value of PoR to indicate the potential to affordrelay for MS. Greater a potential value is, higher probability this pathwill have to act as a relay path. Each RS and MS will maintain a tableto list the values of PoR for different paths. For example, as shown inFIG. 2, RS 1# has two paths to communicate with BSs, which are “RS 1#-BS1#”, and “RS 1#-RS 2#-BS 2#”, therefore, an PoR-path table will bemaintained in a table as below:

TABLE 1 PoR values maintained in RS 1# Recorded Path PoR of the path PoRof the path “RS 1#-BS 1#” “RS 1#-RS 2#-BS 2#” PoR valuePoR_(RS 1#-BS 1#) PoR_(RS 1#-BS 2#)

A generic PoR definition between two stations, A and B, can be writtenas:

$\begin{matrix}{{PoR}_{A\text{-}B} = {{\alpha \cdot ^{- \frac{\prod{k_{n} \cdot \eta_{n}}}{\prod{\lambda_{m} \cdot \gamma_{m}}}}}\mspace{14mu} \left( {{k_{n} > 0};{\eta_{n} > 0};{\lambda_{m} > 0};{\gamma_{m} > 0}} \right)}} & (1)\end{matrix}$

where η_(n), n=1, 2, 3 . . . , is the negative factor, which means thatthe probability of acting relay path will be decreased if this valueincreases. For example, a typical negative factor is path loss. k_(n) isthe corresponding weight for this particular negative factor. γ_(m),n=1, 2, 3 . . . , is the positive factor, which means that theprobability of acting relay path will be increased if this valueincreases. For instance, available bandwidth is a positive factor. λ_(m)is the weight for the m^(th) positive factor. α is the weight for thispath. Some types of path can have higher weight value of α. For example,if we consider latency and signaling overhead, one-hop path can havebigger value of α than multi-hop paths.

If a path is multi-hop, then the PoR of this path is the product of thePoR values of all the links within this path. For example, the PoR valueof α path “A-B-C” can be calculated by:

PoR_(A-B-C)=PoR_(A-B)×PoR_(B-C)  (2)

For example, if we consider path loss, P_(L), and available bandwidth,BW_(a), as the metrics for BS/RS selection, the PoR of link “A-B” can bedefined as:

$\begin{matrix}{{PoR}_{A\text{-}B} = {\alpha \cdot {^{- \frac{P_{L}}{\min {\{{{BW}_{a\_ A},{BW}_{a\_ B}}\}}}}\left( {k > 0} \right)}}} & (3)\end{matrix}$

where P_(L), is the path loss, min{BW_(a) _(—) _(A), BW_(a) _(—) _(B)}is the minimal available bandwidth value of each node in the path. Ifthe pass loss and available bandwidth are listed in FIG. 3, we cancalculate PoR values in RS 1# as:

$\begin{matrix}{{{PoR}_{{RS}\; 1\# \text{-}{BS}\; 1\#} = {{\alpha_{1} \cdot ^{- \frac{25}{\min {\{{0.5,1.5}\}}}}} = {\alpha_{1} \cdot ^{- 50}}}}{{PoR}_{{RS}\; 1\# \text{-}{BS}\; 2\#} = {{\alpha_{2} \cdot ^{{- \frac{22}{\min {\{{1.5,1.5}\}}}} - \frac{15}{\min {\{{1.5,2.5}\}}}}} = {\alpha_{2} \cdot ^{- 25}}}}} & (3)\end{matrix}$

If we simply let α₁=α₂=1, obviously, in terms of the definition of PoR,path “RS 1#-BS 2#” is an optimal path for relay.

RS/BS Selection Method for Handover:

A PoR-path table should be maintained by RSs and MSs.

Step 1: Update PoR of the links between RS/MS and BS

An RS/MS calculates the PoR values of the link between neighboring BSsand itself. The link quality measurement relating to this measurementcan depend on the received preamble, pilot subcarriers, synchronizationsymbols etc.

For example, in FIG. 3, since RS 1# and RS 2# only has one neighboringBS, then the updated PoR-path tables in RS 1#, and RS 2# become:

TABLE 2 PoR table in RS 1# Recorded Path PoR of the path . . . “RS 1#-BS1#” PoR value PoR_(RS 1#-BS 1#) . . .

TABLE 2 PoR table in RS 2# Recorded Path PoR of the path . . . “RS 2#-BS2#” PoR value PoR_(RS 2#-BS 2#) . . .

Step 2: Update PoR of the Paths between an RS/MS and its neighboring RS.

Within this step, an RS shall broadcast its PoR table to its neighboringRS and MS.

Each RS/MS shall update its PoR table in terms of the received PoRtables.

Firstly, an RS will send a PoR-table-broadcast request, PoR_Br_Req, toits BS. BS will allocate slots in downlink subframe for the RSbroadcasting the PoR table. FIG. 4 gives an example of the signalingrelating to PoR table broadcast with WiMAX TDD frame structure:

In FIG. 4. after receiving an PoR_Br_Req, the BS will send an response,PoR_Br_Rsp, to the corresponding RS. The PoR_Br_Rsp packet includes theallocation information of an RS Region in downlink subframe, which canallow RS to broadcast its PoR table and other information.

An RS can broadcast a pre-decided sequence within this RS region, thusother RSs and MSs can use this sequence to measure the link quality. RSscan also broadcast other metrics, such as available bandwidth, withinthis RS region.

An RS/MS will calculate the PoR value of the link between itself and theRS, who broadcasted the PoR table. For example, in FIG. 4, RS 2# cancalculate the PoR_(Rs 2#-RS 1#) in terms of the received information,such as mentioned the available bandwidth, and the pre-decided sequence.Then RS 2# shall multiply PoR_(Rs 2#-RS 1#) with the receivedPoR_(Rs 1#-BS 1#) to obtain the PoR value of the link “RS 2#-RS 1#-BS”,and update its PoR table.

Periodically, an MS shall check the PoR-path table to find an optimalpath with the maximal value of PoR for handover.

Main Benefits

Invention embodiments give an efficient solution for BS/RS selection forMS's handover in WiMAX system. The benefits from this are:

-   -   1. Through the associated improvement in performance (due to        select an optimal BS or RS for handover to guarantee the QoS) to        differentiate FUJITSU's wireless/cable OFDMA (such as WiMAX)        from that of our competitors;    -   2. The proposed method gives an approach of selecting the        optimal BS/RS for handover in non-relay system, and multi-hop        relay system;    -   3. By dynamically selecting an optimal BS/RS, the MS can adapt        to the dynamically changing radio environment and QoS demands;    -   4. A novel metric, named the potential of relay (PoR), is        defined to combine various QoS, or link quality related metrics,        to aid the BS/RS selection and decrease the signaling overhead.        This metric gives a feasible way to calculate the quality of        multi-hop or single-hop links;    -   5. A genetic signaling mechanism is designed for the proposed        selection method, which can be compatible with IEEE802.16e;    -   6. More flexible for distributed implementation. A distributed        implementation can release the computation and signaling load in        BS;    -   7. The definition of RS region gives an feasible method for RSs        to broadcast/transmit information to other RSs, and MSs.

Embodiments of the present invention may be implemented in hardware, oras software modules running on one or more processors, or on acombination thereof. That is, those skilled in the art will appreciatethat a microprocessor or digital signal processor (DSP) may be used inpractice to implement some or all of the functionality of a transmitterembodying the present invention. The invention may also be embodied asone or more device or apparatus programs (e.g. computer programs andcomputer program products) for carrying out part or all of any of themethods described herein. Such programs embodying the present inventionmay be stored on computer-readable media, or could, for example, be inthe form of one or more signals. Such signals may be data signalsdownloadable from an Internet website, or provided on a carrier signal,or in any other form.

1. A path selection method for use in a communication system, the systemcomprising at least three communication apparatuses, a particular one ofsaid apparatuses being operable to transmit and/or receive acommunication signal along at least two different communication paths,each said path being either a single-link path extending from theparticular apparatus to another said apparatus directly over a singlecommunication link therebetween, or being a multi-link path extendingfrom the particular apparatus to said other or another said apparatusindirectly via one or more intermediate said apparatuses over aplurality of consecutive such links link-by-link along the path, and atleast one said path being such a multi-link path, the method comprising:for each link along at least the or one of the multi-link paths,obtaining link-suitability information indicative of the suitability ofthe link concerned for transmission and/or reception; for at least theor said one of the multi-link paths, combining the link-suitabilityinformation for each link of the path concerned so as to generatepath-suitability information indicative of the suitability of the pathconcerned for transmission and/or reception; and selecting one of saidpaths for transmission and/or reception in dependence upon saidpath-suitability information.
 2. The method according to claim 1,wherein said particular apparatus is a user terminal.
 3. The methodaccording to claim 1 or 2, wherein said particular apparatus is a mobileterminal.
 4. The method according to any preceding claim, wherein saidparticular apparatus is a relay apparatus operable to receive acommunication signal over such a communication link and transmit acommunication signal over another such communication link.
 5. The methodaccording to claim 4, wherein the or each intermediate apparatus is arelay apparatus operable to receive a communication signal over such acommunication link and transmit a communication signal over another suchcommunication link.
 6. The method according to any preceding claim,wherein the apparatus at the opposite end of each path to the particularapparatus is a base station apparatus.
 7. The method according to anypreceding claim, wherein at least two of said paths are such multi-linkpaths.
 8. The method according to any preceding claim, furthercomprising: for the or each link of each said path, obtaining suchlink-suitability information, such link-suitability information beingsuch path-suitability information in the case of a single-link path; foreach said multi-link path, combining the link-suitability informationfor each link of the path concerned so as to generate path-suitabilityinformation for the path concerned; and carrying out said selection independence upon the path-suitability information for each said path. 9.The method according to any preceding claim, wherein each obtainedlink-suitability information comprises a numerical factor, and whereinsaid combining comprises mathematically combining the factors concerned.10. The method according to any preceding claim, comprising carrying outsaid selection in the particular apparatus.
 11. The method according toany preceding claim, comprising carrying out said selection inaccordance with predetermined selection criteria.
 12. The methodaccording to claim 11, comprising carrying out said selection byselecting the path having a better suitability than that of the othersaid paths.
 13. The method according to claim 11, comprising carryingout said selection by selecting one of the paths whose path-suitabilityinformation indicates that its suitability is above a predeterminedsuitability threshold.
 14. The method according to any preceding claim,wherein the link-suitability information for at least one of said linkscomprises a combination factor being the result of a mathematicalcombination of a plurality of component factors for that link.
 15. Themethod according to any preceding claim, wherein said system is an OFDMor OFDMA communication system.
 16. A suite of computer programs which,when executed by computing devices of a communication system, causes thesystem to carry out a path selection method, the system comprising atleast three communication apparatuses, a particular one of saidapparatuses being operable to transmit and/or receive a communicationsignal along at least two different communication paths, each said pathbeing either a single-link path extending from the particular apparatusto another said apparatus directly over a single communication linktherebetween, or being a multi-link path extending from the particularapparatus to said other or another said apparatus indirectly via one ormore intermediate said apparatuses over a plurality of consecutive suchlinks link-by-link along the path, and at least one said path being sucha multi-link path, and the method comprising: for each link along atleast the or one of the multi-link paths, obtaining link-suitabilityinformation indicative of the suitability of the link concerned fortransmission and/or reception; for at least the or said one of themulti-link paths, combining the link-suitability information for eachlink of the path concerned so as to generate path-suitabilityinformation indicative of the suitability of the path concerned fortransmission and/or reception; and selecting one of said paths fortransmission and/or reception in dependence upon said path-suitabilityinformation.
 17. A communication system, comprising: at least threecommunication apparatuses, a particular one of said apparatuses beingoperable to transmit and/or receive a communication signal along atleast two different communication paths, each said path being either asingle-link path extending from the particular apparatus to another saidapparatus directly over a single communication link therebetween, orbeing a multi-link path extending from the particular apparatus to saidother or another said apparatus indirectly via one or more intermediatesaid apparatuses over a plurality of consecutive such links link-by-linkalong the path, and at least one said path being such a multi-link path;obtaining means operable, for each link along at least the or one of themulti-link paths, to obtain link-suitability information indicative ofthe suitability of the link concerned for transmission and/or reception;combining means operable, for at least the or said one of the multi-linkpaths, to combine the link-suitability information for each link of thepath concerned so as to generate path-suitability information indicativeof the suitability of the path concerned for transmission and/orreception; and selecting means operable to select one of said paths fortransmission and/or reception in dependence upon said path-suitabilityinformation.
 18. A path selection method for use in a particularcommunication apparatus of a communication system, the system comprisingat least three such communication apparatuses, the particular one ofsaid apparatuses being operable to transmit and/or receive acommunication signal along at least two different communication paths,each said path being either a single-link path extending from theparticular apparatus to another said apparatus directly over a singlecommunication link therebetween, or being a multi-link path extendingfrom the particular apparatus to said other or another said apparatusindirectly via one or more intermediate said apparatuses over aplurality of consecutive such links link-by-link along the path, and atleast one said path being such a multi-link path, the method comprising:for each link or for groups of links along at least the or one of themulti-link paths, obtaining link-suitability information and/orlink-group-suitability information, as the case may be, indicative ofthe suitability of the link or group of links concerned for transmissionand/or reception; for at least the or said one of the multi-link paths,combining the link-suitability information and/or thelink-group-suitability information for the path concerned so as togenerate path-suitability information indicative of the suitability ofthe path concerned for transmission and/or reception; and selecting oneof said paths for transmission and/or reception in dependence upon saidpath-suitability information.
 19. A computer program which, whenexecuted by a computing device of a particular communication apparatusof a communication system, causes the particular apparatus to carry outa path selection method, the system comprising at least three suchcommunication apparatuses, the particular one of said apparatuses beingoperable to transmit and/or receive a communication signal along atleast two different communication paths, each said path being either asingle-link path extending from the particular apparatus to another saidapparatus directly over a single communication link therebetween, orbeing a multi-link path extending from the particular apparatus to saidother or another said apparatus indirectly via one or more intermediatesaid apparatuses over a plurality of consecutive such links link-by-linkalong the path, and at least one said path being such a multi-link path,the method comprising: for each link or for groups of links along atleast the or one of the multi-link paths, obtaining link-suitabilityinformation and/or link-group-suitability information, as the case maybe, indicative of the suitability of the link or group of linksconcerned for transmission and/or reception; for at least the or saidone of the multi-link paths, combining the link-suitability informationand/or the link-group-suitability information for the path concerned soas to generate path-suitability information indicative of thesuitability of the path concerned for transmission and/or reception; andselecting one of said paths for transmission and/or reception independence upon said path-suitability information.
 20. A particularcommunication apparatus of a communication system, the system comprisingat least three such communication apparatuses, the particular one ofsaid apparatuses being operable to transmit and/or receive acommunication signal along at least two different communication paths,each said path being either a single-link path extending from theparticular apparatus to another said apparatus directly over a singlecommunication link therebetween, or being a multi-link path extendingfrom the particular apparatus to said other or another said apparatusindirectly via one or more intermediate said apparatuses over aplurality of consecutive such links link-by-link along the path, and atleast one said path being such a multi-link path, the particularcommunication apparatus comprising: obtaining means operable, for eachlink or for groups of links along at least the or one of the multi-linkpaths, to obtain link-suitability information and/orlink-group-suitability information, as the case may be, indicative ofthe suitability of the link or group of links concerned for transmissionand/or reception; combining means operable, for at least the or said oneof the multi-link paths, to combine the link-suitability informationand/or the link-group-suitability information for the path concerned soas to generate path-suitability information indicative of thesuitability of the path concerned for transmission and/or reception; andselecting means operable to select one of said paths for transmissionand/or reception in dependence upon said path-suitability information.21. The particular communication apparatus according to claim 20, beinga user terminal.
 22. The particular communication apparatus according toclaim 20 or 21, being a mobile terminal.
 23. The particularcommunication apparatus according to claim 20 or 22, being a relayapparatus.
 24. A path selection method for use in a communicationsystem, the system comprising at least three communication apparatuses,a particular one of said apparatuses being operable to transmit and/orreceive a communication signal along at least two differentcommunication paths, each said path being either a single-link pathextending from the particular apparatus to another said apparatusdirectly over a single communication link therebetween, or being amulti-link path extending from the particular apparatus to said other oranother said apparatus indirectly via one or more intermediate saidapparatuses over a plurality of consecutive such links link-by-linkalong the path, and at least one said path being such a multi-link path,the method comprising: for each link along at least the or one of themulti-link paths, obtaining link-suitability information, PoR,indicative of the suitability of the link concerned for transmissionand/or reception, wherein the link suitability information, PoR, foreach link is defined for a link between apparatuses A and B as:${PoR}_{A\text{-}B} = {{\alpha \cdot ^{- \frac{\prod{k_{n} \cdot \eta_{n}}}{\prod{\lambda_{m} \cdot \gamma_{m}}}}}\mspace{14mu} \left( {{k_{n} > 0};{\eta_{n} > 0};{\lambda_{m} > 0};{\gamma_{m} > 0}} \right)}$where η_(n), n=1, 2, 3 . . . , is a negative factor, k_(n) is thecorresponding weight for the n^(th) negative factor, γ_(m), m=1, 2, 3 .. . , is a positive factor, λ_(m), is the weight for the m^(th) positivefactor, and α is the weight for the link from apparatus A to apparatusB; for at least the or said one of the multi-link paths, combining thelink-suitability information for each link of the path concerned bymultiplying the PoR for each of those links together so as to generatepath-suitability information indicative of the suitability of the pathconcerned for transmission and/or reception; and selecting one of saidpaths for transmission and/or reception in dependence upon saidpath-suitability information.